Answer: a. the number of electron shells increases.
Explanation:
The elements in the vertical columns also called as groups are arranged in order of increasing atomic numbers and contain same valence electrons.
The electrons are added to new energy shells and the nuclear charge also increases due to increase in atomic number. But the effect of addition of new shells is more dominant.
As the shells increase, the atomic size increases and the valence shell moves farther from the nucleus , thus the tendency to lose electrons increase and thus the ionization energy which is the energy required to move the valence electron, decreases.
